MESSENGER is a scientific investigation of Mercury, the least explored of the terrestrial rocky planets that also include Venus, Earth and Mars. Understanding Mercury and how it formed is critical to better understanding the conditions on and evolution of the inner planets. The project takes advantage of clever mission designs, tougher lightweight materials and miniature technologies all developed in the three decades since Mariner 10 flew past Mercury. The compact orbiter, fortified against the searing conditions near the Sun, will investigate key questions about Mercury's characteristics and environment with a set of seven scientific instruments.

MESSENGER matched Mercury's orbit with a series of flybys of Earth, Venus and Mercury, using gravity to adjust its path each time. Three Mercury flybys, which included photographing and measurements of the planet's previously unseen side, provided information critical to the planning and carrying out of the orbital study of the innermost planet.

MESSENGER lifted off from Cape Canaveral Air Force Station, Fla., on a three-stage Boeing Delta II expendable launch vehicle on August 3, 2004.

MESSENGER's path to Mercury took it past Venus twice. The spacecraft used the tug of Venus' gravity to resize and rotate its trajectory closer to Mercury's orbit. On Oct. 24, 2006, MESSENGER flew past Venus at a distance of about 2,987 kilometers (1,856 miles), reducing its orbit's perihelion and aphelion (minimum and maximum distance from the Sun) and increasing the orbit inclination - the tilt angle relative to Earth's orbit around the Sun. The approaching spacecraft viewed a brightly sunlit Venus. The departure view was mostly dark .

The spacecraft's second Venus flyby was on June 5, 2007 at a distance of 338 kilometers (210 miles), moving a little closer to the farthest point in Mercury's orbit. This pass increased the inclination of MESSENGER's orbit to match that of Mercury.

Aside from course corrections, the flybys gave MESSENGER engineers and scientists a chance to check out the instruments and practice observing techniques for the three Mercury flybys.

Three 200-kilometer (124-mile) minimum-altitude Mercury flybys, each followed about two months later by a course-correction maneuver, put MESSENGER in position to enter Mercury orbit in mid March 2011. MESSENGER will map nearly the entire planet in color, image most of the areas unseen by Mariner 10, and measure the composition of the surface, atmosphere and magnetosphere. It will be the first new data from Mercury in more than 30 years - and invaluable for planning MESSENGER's yearlong orbital mission.

For one year MESSENGER will operate in a highly elliptical (egg-shaped) orbit around Mercury, 200 kilometers (124 miles) above the surface at the closest point and 15,193 kilometers (9,420 miles) at the farthest. The plane of the orbit is inclined 80 degrees to Mercury's equator, and the low point in the orbit comes at 60 degrees north latitude. MESSENGER will orbit Mercury twice every 24 hours; for eight of those hours it will be oriented for sending data to Earth.

Orbit insertion occurs on March 18, 2011. Using 30 percent of its propellant, MESSENGER will fire its large thruster and slow down by just about 0.9 kilometers (about a half mile) per second, coming to a virtual stop relative to Mercury. The first orbit insertion maneuver (lasting about 14 minutes) places the spacecraft into a stable orbit; it also sets up a much shorter "cleanup" maneuver two days later near the orbit's lowest point.

MESSENGER's 12-month orbit covers two Mercury solar days; one Mercury solar day, from noon to noon, is equal to 176 Earth days. MESSENGER will obtain global mapping data from the different instruments during the first day and focus on targeted science investigations during the second. While MESSENGER circles Mercury, solar gravity and radiation will slowly and slightly change the spacecraft's orbit. Once every Mercury year (or 88 Earth days) MESSENGER will carry out a pair of maneuvers to reset the orbit to its original size and shape.

After Mariner 10's visits to Mercury the space science and engineering communities yearned for a longer and more detailed look at the innermost planet - but that closer look, ideally from orbit, presented formidable technical obstacles. A Mercury orbiter would have to be tough, with enough protection to withstand searing sunlight and roasting heat bouncing back from the planet below. The spacecraft would need to be lightweight, since most of its mass would be fuel to fire its rockets and slow the spacecraft down enough for Mercury's gravity to capture it. And it would have to be compact enough to lift off on a conventional and cost-effective rocket.

MESSENGER spacecraft tackles each of these challenges. A ceramic-fabric sunshade, heat radiators and a mission design that limits time over the planet's hottest regions protect MESSENGER without expensive and impractical cooling systems. The spacecraft's graphite composite structure - strong, lightweight and heat tolerant - is integrated with a low-mass propulsion system that efficiently stores and distributes the approximately 600 kilograms (1,323 pounds) of propellant that accounts for 55 percent of MESSENGER's total launch weight.

To fit behind the 8-foot by 6-foot sunshade MESSENGER's wiring, electronics, systems and instruments are packed into a small frame that could fit inside a large sport utility vehicle. And the entire spacecraft is light enough to launch on a Delta II rocket, the largest launch vehicle allowed under NASA's Discovery Program of lower-cost space science missions.

While orbiting Mercury, MESSENGER will "feel" significantly hotter than spacecraft that orbit Earth. This is because Mercury's elongated orbit swings the planet to within 46 million kilometers (29 million miles) of the Sun, or about two-thirds closer to the Sun than Earth. The Sun also shines up to 11 times brighter at Mercury than we see from our own planet.

MESSENGER's first line of thermal defense is a heat-resistant and highly reflective sunshade, fixed on a titanium frame to the front of the spacecraft. Measuring about 2.5 meters (8 feet) tall and 2 meters (6 feet) across, the thin shade has front and back layers of Nextel ceramic cloth - the same material that protects sections of the space shuttle - surrounding several inner layers of Kapton plastic insulation. While temperatures on the front of the shade could reach 370 degrees C (698 degrees F) when Mercury is closest to the Sun, behind it the spacecraft will operate at room temperature, around 20 degrees C (68 degrees F). Multilayered insulation covers most of the spacecraft.

Radiators and one-way heat pipes are installed to carry heat away from the spacecraft body, and the science orbit is designed to limit MESSENGER's exposure to heat re-radiating from the surface of Mercury. (MESSENGER will only spend about 25 minutes of each 12-hour orbit crossing Mercury's broiling surface at low altitude.) The combination of the sunshade, thermal blanketing and heat-radiation system allows the spacecraft to operate without special high-temperature electronics.